The present invention relates in general to a thin film transistor-liquid crystal display (hereinafter "TFT-LCD"), and more particularly to a signal line structure for TFT-LCD and a method for fabricating the same, capable of reducing the resistance of the signal line and improving the production yield.
Hereinafter, a conventional TFT-LCD and a fabrication method will be described along with the problems generated thereby with reference to several drawings.
Referring initially to FIG. 1, there is shown a signal line structure for TFT-LCD which was reported in "Japan display (89), Kyoto" pp. 498 to Ikeda.
As shown in FIG. 1, the TFT-LCD structure is formed of a glass substrate 1 which comprises a first metal (tantalum film) 2, a second metal (copper film) 3 and a third metal (tantalum film) 4 which are piled to form signal lines, that is, a gate electrode 13a and a data line 13b, along with an insulating film 5 covering the signal lines, an amorphous silicon layer 6, an n.sup.+ amorphous silicon layer 7 and a source/drain electrode 8 that are formed over the gate electrode 13a, in due order.
In the meanwhile, the field of the invention is the signal lines. Of the components consisting of the signal lines, the first metal film 2 plays a role in improving the adhesiveness to the glass substrate 1. The second metal film 3, another component, allows the resistance to be reduced to, for example, about 3 .mu..OMEGA..multidot.cm, whereas the third metal film 4, another component, is formed in order to prevent the oxidation of the second metal film which is vulnerable to oxidation.
Referring now to FIG. 2, there is illustrated a conventional method for fabricating the signal line for TFT-LCD. First, over a glass substrate 1, a first metal (tantalum) film 2 with a thickness of approximately 500 .ANG. is deposited by sputtering, followed by the deposition of a second metal (copper) film 3 and a third metal (tantalum) film 4 which are approximately 2,000 and 500 .ANG. thick, respectively, over the first metal film 2 in due order, as shown in FIG. 2A.
Subsequently, on the third metal film 4 is deposited a photoresist 9 which is, then, subjected to photo lithography to define the width of signal line, and the third metal film 4 is subjected to the treatment of dry etching under CF.sub.4 /O.sub.2 gas to expose the second metal film 3, as shown in FIG. 2B.
Next, the exposed second metal 3 is subjected to a wet etching process in an acetic acid type solution and the first metal 2 is treated in a manner similar to that of the third metal 4 under CF.sub.4 /O.sub.2 gas, as shown in FIG. 2C.
Finally, the photoresist 9 is removed and an insulating film 5 is formed in a thickness of approximately 5,500 .ANG. by a plasma chemical vapor deposition process, so as to fabricate a signal line for TFT-LCD, as shown in FIG. 2D.
However, there occurs several problems in performing the above conventional method. Particularly, when the wet etching is applied to the second metal 3, subsequent to the dry etching for the third metal 4 as shown in FIG. 2C, the side surface of the second metal 3 is etched as shown in FIG. 3A, which is a partially enlarged detailed view, since the wet etching, in principle, has the same etching speed for a vertical and a horizontal direction. In addition, since the signal line insulating film 5 is formed of a silicon oxide film under an oxidative atmosphere, the second metal 3 is oxidized, so that the volume of the second metal is expanded, which results in bending of the third metal 4, as shown in FIG. 3B, which is another enlarged, detailed view of the A part of FIG. 2C.
The deformed structure causes to generate a leakage between the gate electrode and the source/drain electrode in a thin film transistor fabricated. In the worst case, there occurs a shortage phenomenon between the gate electrode and the source/drain electrode, so that the quality of the TFT-LCD becomes defective.
For preventing the shortage between the gate electrode and the source/drain electrode, the gate electrode is subjected to the treatment of anode oxidation to form an anode oxide film, or an insulating film, which is structured to comprise double components of a silicon oxide film and a silicon nitride film. However, application of anode oxidation to the signal line which is structured to comprise treble components of the first metal 2, the second metal 3 and the third metal 4 as shown in the conventional structure is accompanied by the anode oxidation of the upper third metal 4. At this time, the side portion of the second metal 3 is not oxidized. In addition, it is placed under such a danger that it may be corroded by a solution for anode oxidation. Consequently, even this method is not enough to prevent the leak current between gate electrode and the source/drain electrode from being generated in a TFT-LCD and to improve the production yield.